177
Determining External Regen Resistor Size
Power
Dissipation
Calculation
Procedure
The following is the procedure for calculating the power dissipated by the Regen
resistor in a simple system wherein friction is negligible. Ignoring friction in the
following calculations gives worst case results since friction will absorb a portion of
the energy during deceleration. An example of each step in this procedure is
provided later in this chapter.
Continued on next page
Step Action
1 Plot speed versus time and torque versus time for the entire move cycle.
(Magnitude of the torque is not required; only the direction is required.)
2 Identify each section of the plot where the drive is decelerating the load or where
speed and torque have opposite signs.
3 Calculate the energy returned to the drive in each deceleration using the formula E
= ½ J
t
ω
2
Where
E = Energy in joules
J
t
= Total system inertia, including motor, in kg(m
2
)
ω
= Speed at start of deceleration in radians per second
(ω
= 2
π
RPM / 60)
4
Compare the energy in each deceleration with the energy required to turn on the
Regen circuit. (See Drive Energy Absorption Capability table.) If the energy is less
than that listed in the table, disregard that deceleration for the remainder of the
calculations.
5
Calculate the energy dissipated by the Regen resistor by subtracting the energy
listed in the table from the energy of the deceleration.
E
dissipated
= E
generated
– E
absorbed by capacitors
6 Calculate the pulse power of each deceleration by dividing the dissipated energy
by the time of the deceleration.
P
pulse
= E
dissipated
/ T
decel (seconds)
7 Calculate the continuous power dissipated by the Regen resistor by totaling all the
dissipated energy and dividing it by the total cycle time.
P
continuous
= (E1
dissipated
+ E2
dissipated
+… +En
dissipated
) / T
total cycle (seconds)
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